Boiling water nuclear reactors have fuel bundles of well known construction. The fuel bundles each include a lower tie plate for supporting a matrix of upstanding fuel rods and for inletting moderating water coolant. The fuel bundle includes an array of upstanding fuel rods at least some of which extend to an upper tie plate. The upper tie plate serves to hold the fuel bundle together through at least some of the fuel rods and to permit the outflow of water and generated steam. To prevent abrading contact between dynamically vibrating long (about 160 inches) and relatively thin fuel rods, so-called spacers are placed at selected vertical elevations around the fuel rods within the fuel bundle. Finally, the entire assembly is enclosed by a channel, this channel extending from the vicinity of the lower tie plate to the vicinity of the upper tie plate.
The fuel bundle construction is essentially a square of approximately 5.3 by 5.3 inches interior dimension. It is bounded by a correspondingly square channel. The normal channel construction includes four rounded corners and four flat sides extending there between. The fuel bundles--placed within their square channels--are arrayed in parallel upstanding relation one to another with their respective flat channel sides in parallel alignment one to another. This being the case, the fuel bundles define between their channels a cruciform shaped interstitial volume. It is into this cruciform shaped interstitial volume that cruciform shaped control rods penetrate for controlling the nuclear reaction.
The channels serve to define two flow regions in the reactor core. One flow region is through the fuel bundles for the generation of steam from which power is ultimately extracted. The other flow region is the so-called core bypass region and includes the cruciform interstitial region defined between the fuel bundles. This region is exterior of the fuel bundle channels and occupies the region interstitially of the fuel bundles which is penetrated by the reactor control rods. When the control rods are inserted, thermal neutrons are absorbed and the nuclear reaction is abated. When the control rods are withdrawn, water within the core bypass region replaces the control rods and enhances neutron moderation which in turn promotes the nuclear reaction.
Typically, and when fuel bundles are expended, only the fuel rods, tie plates and spacers are replaced. Channels can be replaced into a reactor and last for more than one fuel bundle lifetime. These channels are limited in their life time by channel "bow" and channel "bulge."
Channel "bow" is the tendency of the entire channel to warp in one direction or another responsive to radiation gradient across the entire channel. Such bow is a function of many factors including whether the reactor has uneven fuel bundle spacing (as in so-called D-lattice constructions) or even fuel bundle spacing (as in so-called C-lattice constructions). Further, channel bow can be a function of where in the reactor core the fuel bundle is located. Because this condition is a function of overall fuel bundle radiation gradient, it is not subject to correction by the modifications of this disclosure.
Channel bulge is the tendency of the flat sides of the fuel bundle to become tubular. This bulge phenomena is subject to reduction by the disclosure herein. Accordingly, it will suffice to review the factors which cause channel bulge so that the tendered design which improves resistance to this phenomena can be understood.
Channel bulge is the result of a combination of two conditions working upon the flat sides of the channels. One condition is the force of the pressure differential acting between the relatively high pressure region in the interior of the fuel bundle and the relatively low pressure region exterior of the fuel bundle channel in the core bypass region. The other condition is the ambient neutron radiation. A discussion of these two effects can lead to an understanding of channel bulge phenomenon.
Fluid flow of the coolant through the fuel bundle is promoted by a pressure differential. This pressure differential includes a relatively high pressure region in the bottom of the fuel bundle changing slowly to a lower pressure region in the upper two phase region of the fuel bundle at the discharge of the fuel bundle flow path through the upper tie plate. This pressure differential is needed to overcome the forces of fluid friction as the water moderator passes between the fuel rods and through the spacers. Further, the forces of acceleration consume pressure drop as the relatively slower moving liquid moderator turns into relatively faster moving generated steam.
In modern fuel bundle design, this pressure resistance to flow interior of the fuel bundle has become aggravated. Specifically, and in order to realize maximum nuclear and thermodynamic efficiencies, the density of the arrays of fuel rods within modern boiling water fuel bundles has increased. This increase has in turn lead to greater pressure drop requirements for assuring the necessary flow through the fuel rods and spacers interior of the fuel bundle.
Exterior of the fuel bundle channels and in the core bypass region another lower pressure regime exists. Water moderator is normally discharged directly to the core bypass region. In this region the water does not appreciably boil and has essentially a low resistance flow path. Consequently, the core bypass region is a zone of low pressure.
The fuel bundle channels residing in this pressure domain thus have a relatively high pressure interior and a low pressure exterior. Accordingly, the flat sides of the fuel bundle channels come under stress. This stress--if completely relaxed--would cause the fuel bundle channels to become tubular or cylindrical. In any event, the flat sides of the fuel bundles have the tendency to bulge in response to the differential pressure.
Unfortunately, the ambient radiation has the effect of relaxing the stress within the channels due to a phenomenon called "radiation induced creep". This relaxing of stress causes an elastically deformed and bulged channel to retain its bulge. Accordingly, and with time, these bulges both increase in dimension and become permanent. Increase in dimension continues until the bulged sides either interfere with the required control rod travel or alternatively effect the nuclear and thermodynamic performance of the fuel bundle. Thereafter, the channels must be replaced. Naturally, any design which prolongs the tendency of the fuel bundle channels to resist this bulge phenomena is desirable.
It will be further understood that bulge is a combination of the pressure differential force--which is at a maximum at the bottom of the fuel bundle--and radiation--which is at a maximum roughly in the middle of the fuel bundle. As these two effects combine, experience has shown that the bulge phenomena has its maximum effect at the lower 1/3 to 1/4 of a fuel bundle.
The increased density of the fuel bundle array has had another adverse effect on fuel bundles. This effect is related to the increase in fuel rod pitch--or the decrease in fuel rod interstitial separation and fuel rod to channel spacing across the fuel bundle. This decrease in fuel rod to fuel rod spacing or fuel rod to channel spacing aggravates the critical power of certain fuel rods within the fuel bundle--especially those fuel rods which are in the corner locations.
Critical power is defined as that power output of a fuel rod that causes boiling transition--the "drying out" of the water layer covering a fuel rod--to occur. The point at which boiling transition occurs is effected by the fluid flow occurring over the surface of the fuel rod. As a general proposition, a greater moderator flow over the surface of the fuel rod permits a larger power output of the fuel rod before the limits of critical power are exceeded.
A dense fuel array aggravates the critical power phenomena in at least two ways. First, as the total number of fuel rods within the fuel bundle cross section increases, the spacing between the fuel rods must inevitably decrease. When the spacing between the fuel rods decreases, the critical power of the fuel rods is generally decreased.
Secondly, the spacing between the fuel rods and the surrounding channel decreases. This is especially true at the corner locations. When it is remembered that fuel rods at these locations are adjacent--on at least two sides--to the greater moderator density of the core bypass region, it will be understood that these fuel rods have a tendency to have a relatively greater power output. Consequently, the restriction of the flow area between these fuel rods and the channel at the corner locations causes these fuel rods to be limiting in their critical power performance.
The reader will also remember that fuel rod spacers are placed at selected elevations along the fuel bundle. This being the case, further restriction of flow occurs at the fuel rod spacers--again especially in the corner locations.
It will further be understood that the construction of the fuel bundle channels--and especially the thickness of the channel walls--is an art which compromises two competing phenomena. The first phenomena is the ability of the fuel channel to resist bulge. The second phenomena is the parasitic absorption of neutrons by the metal which makes up the fuel bundle channel. Consequently, and as disclosed in Crowther et al. U.S. Pat. No. 4,749,544 entitled THIN WALLED CHANNEL issued Jun. 7, 1988 (now Reissue U.S. Pat. No. RE 33,818, allowed Aug. 7, 1992), it is known to vary the construction of the thickness of the channel wall as a function of the stress encountered locally by the channel wall. Where stresses are small, the channel wall is of a relatively thin construction. Where the stresses are large, the channel wall is of a thicker construction.
The reader will appreciate that the above considerations are selected reactor core phenomena which are applicable to understanding this invention. The combined relevance of these consideration is necessary to understand the following fuel bundle design.